build with zig master also

`aarch64_32` has ceased to exist. Actually, I'm not sure it should have
been used on this switch prong anyway.

.github/workflows/ci.yaml

@@ -20,6 +20,7 @@ jobs:
           - zig-version: "0.12.1"
             os: ubuntu-latest
           - zig-version: "master"
+            check-format: true
             os: ubuntu-latest
     runs-on: ${{ matrix.os }}
     steps:
@@ -31,7 +32,8 @@ jobs:
         with:
           version: ${{ matrix.zig-version }}
 
-      - name: Check Formatting
+      - if: ${{ matrix.check-format }}
+        name: Check Formatting
         run: zig fmt --ast-check --check .
 
       - name: Build